Set a medium saucepan over moderate heat and sauté the diced onion in olive oil until softened and aromatic; about 8-10 minutes. Add the sage, frozen Wild Blueberries, vinegar, salt, pepper, and nutmeg if desired. Mix to combine and cook, stirring periodically, until the Wild Blueberries have mostly burst and their juices have reduced. This should take about 15-20 minutes. Use the side of your spoon or spatula to lightly mash the berries to break them down further.
Mix together the arrowroot and water separately to create a slurry before adding it to the pan. Stir thoroughly and cook for just a minute or two longer until the mixture has visibly thickened. Let jam get to room temperature before either storing in a glass container or serving.